Magnetic recording media generally include a binder dispersion layer comprising a binder and a pigment overlying a substrate, wherein the pigment is dispersed within the binder. Typically, the pigment is a magnetizable pigment comprising small, magnetizable particles. In some instances, the medium may be in the form of a composite having both back-coat and front-coat binder dispersion layers, although the pigment in the back-coat may or may not be a magnetizable pigment.
It has become desirable to have as high a loading of magnetizable pigment in the magnetic recording media as is reasonably possible. It is often preferred to have a binder dispersion comprising from about 70% to 85% by weight magnetizable pigment relative to the binder with as many magnetizable particles per unit area or unit volume as possible. It is also preferred to have a binder dispersion in which the magnetizable pigment comprises a plurality of small particles having a relatively high specific surface area. Higher pigment loading has the potential to provide high density magnetic recording media capable of storing more information.
Problems, however, remain in the art concerning magnetic recording media having a relatively high loading of magnetizable pigment. To begin with, magnetizable pigments tend to agglomerate, and they are difficult to properly and fully disperse within the binder. Wetting agents, or dispersants, are often employed to facilitate such dispersion. For higher pigment loading, i.e., the use of greater amounts by weight and number of magnetizable particles, greater amounts of such dispersants are required, which is not always desirable. There are a number of reasons for using as little dispersant as possible. Costs, for example, can be reduced by using less dispersant. Additionally, binder dispersions can be more readily and reproducibly prepared when less dispersant is used. Further, excess dispersant may have a tendency to bloom from a cured binder dispersion over time, leading to contamination of a recording head or the like, or causing a change in the physical or chemical characteristics of the media.
Another problem in the art is that the viscosity of a binder dispersion generally increases with higher loading of magnetizable pigment. If the dispersion is too viscous, it can be difficult to apply to the substrate, and good magnetic orientation of the pigment, i.e., a squareness ratio of 0.75 or more, can be hard to obtain. The squareness ratio (Br/Bm), which is the ratio of the remnant saturation induction, or residual magnetization (Br), to the saturation induction, or saturation magnetization (Bm), refers to the effectiveness of the orientation of the magnetic particles. For randomly-oriented particles, the squareness ratio is 0.5 and for ideally and perfectly oriented particles, the ratio is equal to 1.0. Values for the squareness ratio, of media exhibiting good performance, normally fall around 0.75 to 0.85, with higher values being better.
To help alleviate these problems with high pigment loading, binder compositions having internal dispersants have been developed. Such compositions comprise polymers with functional moieties pendant from the polymer backbone that help disperse the magnetizable pigment. As a result of using these compositions, less dispersant is needed for dispersion of magnetizable pigment in the binder.
Copending U.S. application Ser. No. 07/429,720, filed Oct. 31, 1989 now U.S. Pat. No. 5,071,578 which is a continuation-in-part of U.S. application Ser. No. 07/295,046, filed Jan. 6, 1989 now abandoned (assigned to the assignee of the present case) discloses a curable polymer having internal dispersants which comprises a sulfonated, hydroxy-functional polyurethane (SHPU) resin. The SHPU resin can be used as part of a binder layer in magnetic recording media. The application discloses the excellent dispersion of magnetizable pigment within the SHPU resin and the ease of application of the dispersion onto a suitable substrate.
However, there are certain types of media constructions where high glass transition temperature (Tg) and abrasion resistance in the uncured state, and high final coating modulus are both desirable and necessary, for example, where stiffer tapes are needed for better handling in the recording and playback machine, or where manufacturing methods dictate the necessity of having high initial strength coatings that are resistant to damage during manufacture or processing.
In order to increase the durability, running properties, and reliability of the magnetic recording media, attempts have been made to add a hard material to the pigment-containing binder.
U.S. application Ser. No. 07/315,304 now abandoned (assigned to the assignee of the present case), incorporated by reference herein, discloses a blend of SHPU and another polymer usually called a "hard resin" which is used to attain the desired high T.sub.g for optimum binder performance in certain applications. The "hard resin" disclosed in 07/315,304 is a cross-linkable hydroxy-functional vinyl chloride having incorporated therein --SO.sub.3 M groups wherein M represents H.sup.+ or a metal cation. Although polymer blends of the type disclosed in 07/315,304 are very useful the dispersing character of the binder system can sometimes diminish. In addition, only a few types of hard resins have been found to be functional because of a general lack of polymer/polymer compatibility. For example, hard resins based upon acrylic monomers have not usually been found to be usefully compatible with the preferred types of polyurethane binder resins.
In order to optimize binder performance, single polymer systems have been developed in which a "hard resin" is grafted onto a polyurethane having hydroxy and sulfonate groups rather then blended therewith.
Copending concurrently filed U.S. patent application Anderson et al., Ser. No. 07/543,230 now U.S. Pat. No. 5,134,035 (assigned to the assignee of the present case) describes sulfonated and non-sulfonated, hydroxy and thiol functional polyurethanes and graft copolymers made therewith prepared via free radical polymerization of vinyl monomer in the presence of sulfonated and non-sulfonated, hydroxy, and thiol functional polyurethanes, respectively. Although the method has a number of distinct advantages including the ability to use a broad range of vinyl monomers the method does not provide for the use of anionically polymerized vinyl monomer segments and is limited to an extent regarding synthetic control.
Copending concurrently filed U.S. patent application Kumar et al., Ser. No. 07/543,361, now U.S. Pat. No. 5,118,580 (assigned to the assignee of the present case) discloses dithiocarbamate modified sulfonated and non-sulfonated hydroxy-functional macroiniferter compounds and vinyl/sulfonated and non-sulfonated hydroxy functional polyurethane (vinyl/SHPU and vinyl/HPU) graft and block copolymers made therewith using photoiniferter technology. The method generates quality material which performs very well when tested in magnetic recording applications. However, due to the photopolymerization required, the acquisition of special manufacturing equipment is necessary.
Thus, somewhat similar copolymers can be prepared via the above two methods although each method and resultant copolymer has its own distinct advantages. Although the above two methods are extremely useful a need exists for a method which offers simplicity, practicality, superior synthetic control, and economic efficiency regardless of the range of possible compositions. A need thus exists for a single polymer magnetic binder system which does not need to be blended with a hard resin which possesses a high T.sub.g, good mechanical properties, disperses pigment well, and which is capable of being crosslinked by isocyanate curatives. We have discovered such a copolymer and method.